High-resolution Brillouin analysis of composite materials beams Yosef London a , Yair Antman a , Maayan Silbiger b , Liel Efraim a , Avihay Froochzad a , Gadi Adler b , Eyal Levenberg c , Avi Zadok a,* a Faculty of Engineering, Bar-Ilan University, Ramat-Gan 5290002, Israel; b Xenom Ltd., 4 Gluska st., Rishon LeTzion 7565004, Israel; c Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, Haifa 3200003, Israel. ABSTRACT High-resolution Brillouin optical correlation domain analysis of fibers embedded within beams of composite materials is performed with 4 cm resolution and 0.5 MHz sensitivity. Two new contributions are presented. First, analysis was carried out continuously over 30 hours following the production of a beam, observing heating during exothermal curing and buildup of residual strains. Second, the bending stiffness and Young's modulus of the composite beam were extracted based on distributed strain measurements, taken during a static three-point bending experiment. The calculated parameters were used to forecast the beam deflections. The latter were favorably compared against external displacement measurements. Keywords: Stimulated Brillouin scattering, optical fiber sensors, structural health monitoring, composite materials, distributed sensors, strain measurements. 1. INTRODUCTION Distributed fiber-optic sensors of temperature and strain, based on stimulated Brillouin scattering (SBS), are known for over 25 years 1,2 . Commercial SBS analyzers provide a measurement range of tens of kilometers and sub-meter spatial resolution. They are deployed primarily in the monitoring of long, critical infrastructure in the oil and gas and energy sectors. The adaption of Brillouin analysis by other areas of application, such as the construction, transportation and aerospace sectors, had been slowed down by spatial resolution limitations of most analysis protocols 3 . Much effort is being dedicated to resolution enhancement in Brillouin sensing setups towards cm- and even mm-scale, using both time- domain and correlation-domain analysis 4-8 . Our group and coworkers proposed a Brillouin optical correlation-domain analysis (B-OCDA) protocol, in which the pump and signal waves are jointly modulated by carefully-constructed, high- rate binary phase sequences 9-11 . The technique enabled the Brillouin analysis of a 2.2 km-long fiber under test, with a spatial resolution of 2 cm 11 . Composite materials consist of strength members such as glass or carbon fibers, embedded in a polymer matrix. Composites provide favorable ratios of strength to weight, corrosion resistance, and freedom in forming complex three- dimensional shapes. Many critical parts in the aerospace, energy, transportation and medical sectors are made of composite materials. The consequences of failure in such parts could be catastrophic, a situation that warrants reliable structural monitoring. In most cases, static and dynamic structural analysis of composite material parts relies on point sensors, such as strain gauges, accelerometers or fiber Bragg gratings (FBGs). In a series of works by the Tur and collaborators, FBGs were used in the monitoring of curing processes of composite patch repairs 12 and in measurement of Lamb waves 13 . Distributed Brillouin sensing was introduced to monitoring in aircrafts in 2001 14 , and the analysis of full- scale composite structures with 1 m resolution was reported in 2003 15 . Minardo et al. recently used dynamic Brillouin sensing in vibration modal analysis of a composite structure, with a spatial resolution of 20 cm 16 . Peled and coworkers used slope-assisted Brillouin optical time domain analysis in the dynamic monitoring of flexural waves in a 20 m-long composite strip 17 . In this work, we employ high-resolution Brillouin analysis to the monitoring of beams made of composite materials. Standard single-mode sensing fibers were embedded during fabrication of the beams. The spatial resolution of the measurements was between 4 and 8 cm, and their sensitivity was 10 µε. The novelty of the analysis is in two respects: First, the strain along the beam was continuously monitored following its production, over more than 30 hours. The * Avinoam.Zadok@biu.ac.il Phone: +972-3-5318882; Fax: +972-3-7384051; http://www.eng.biu.ac.il/~zadoka 24th International Conference on Optical Fibre Sensors, edited by Hypolito José Kalinowski, José Luís Fabris, Wojtek J. Bock, Proc. of SPIE Vol. 9634, 96346N · © 2015 SPIE CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2194521 Proc. of SPIE Vol. 9634 96346N-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/03/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx